Knob for tuning a guitar string



FIG. 1 is a perspective view of a knob for tuning a guitar string, showing my new design;

FIG. 2 is a front view thereof;

FIG. 3 is a rear view thereof;

FIG. 4 is a top view thereof, the bottom view being identical;

FIG. 5 is a left side view thereof, the right side view being a mirror image thereof; and,

FIG. 6 is a reduced top view of a knob for tuning a guitar string, the broken lines shown are for illustrative purposes only and form no part of the claimed design. 

The ornamental design for a knob for tuning a guitar string, as shown and described. 